In case of collision the airbag will inflate with gas to absorb the energy from the occupants, increasing the pressure inside the airbag. To counteract this increase in pressure it is common to provide vent apertures directly in the airbag fabric.
It would be desirable for these apertures to be absent or to be very small until there is a significant pressure increase in the airbag.
In this sense, several solutions are known that allow releasing the gas in case of an excessive pressure increase in the airbag.
For example, U.S. Pat. No. 7,782,325, published on 9 Nov. 2010, describes an airbag provided with a vent aperture covered by a membrane that in turn defines an orifice with a diameter smaller than that of the vent aperture. When the pressure in the airbag increases, the diameter of said orifice in the membrane also increases, allowing the gas to be released and reducing the pressure inside the airbag.
This solution requires the periphery of said orifice to be defined by a region of increased membrane thickness, in order to minimise the risk of the membrane material breaking due to the pressure increase. However, this thickening of the internal edge of the membrane prevents the diameter of the orifice from increasing enough to efficiently counteract the overpressure in the airbag at the time it occurs.
In addition, U.S. Pat. No. 7,740,274, published on 22 Jun. 2010, discloses an airbag that includes a vent aperture allowing the gas contained in the airbag to be released upon impact. It comprises a complex series of membranes and/or a diaphragm disposed on the vent aperture that control the amount of gas released based on the pressure in the airbag.
Patent EP 1575810, published on 9 May 2007, discloses an airbag with a vent orifice covered by two ring-shaped pieces provided with radial slits in the area that is meant to cover the orifice partially, and a membrane for sealing said orifice placed between them. The sealing membrane is meant to break when the pressure in the airbag reaches a predetermined value.
In this solution, once the membrane has been broken at the time of the overpressure, it is no longer possible to reduce the size of the vent orifice created in said sealing membrane when the pressure inside the airbag is reduced.